title: Crystal structure of Ca2+/H+ antiporter protein YfkE reveals the mechanisms of Ca2+ efflux and its pH regulation.
abstract: Ca(2+) efflux by Ca(2+) cation antiporter (CaCA) proteins is important for maintenance of Ca(2+) homeostasis across the cell membrane. Recently, the monomeric structure of the prokaryotic Na(+)/Ca(2+) exchanger (NCX) antiporter NCX_Mj protein from Methanococcus jannaschii shows an outward-facing conformation suggesting a hypothesis of alternating substrate access for Ca(2+) efflux. To demonstrate conformational changes essential for the CaCA mechanism, we present the crystal structure of the Ca(2+)/H(+) antiporter protein YfkE from Bacillus subtilis at 3.1-Å resolution. YfkE forms a homotrimer, confirmed by disulfide crosslinking. The protonated state of YfkE exhibits an inward-facing conformation with a large hydrophilic cavity opening to the cytoplasm in each protomer and ending in the middle of the membrane at the Ca(2+)-binding site. A hydrophobic "seal" closes its periplasmic exit. Four conserved α-repeat helices assemble in an X-like conformation to form a Ca(2+)/H(+) exchange pathway. In the Ca(2+)-binding site, two essential glutamate residues exhibit different conformations compared with their counterparts in NCX_Mj, whereas several amino acid substitutions occlude the Na(+)-binding sites. The structural differences between the inward-facing YfkE and the outward-facing NCX_Mj suggest that the conformational transition is triggered by the rotation of the kink angles of transmembrane helices 2 and 7 and is mediated by large conformational changes in their adjacent transmembrane helices 1 and 6. Our structural and mutational analyses not only establish structural bases for mechanisms of Ca(2+)/H(+) exchange and its pH regulation but also shed light on the evolutionary adaptation to different energy modes in the CaCA protein family.
authors.affiliation: Center for Membrane Biology, Department of Biochemistry and Molecular Biology, University of Texas Houston Medical School, Houston, TX 77030, USA.
authors.forename: Mousheng
authors.initials: M
authors.lastname: Wu
authors.forename: Shuilong
authors.initials: S
authors.lastname: Tong
authors.forename: Sandro
authors.initials: S
authors.lastname: Waltersperger
authors.forename: Kay
authors.initials: K
authors.lastname: Diederichs
authors.forename: Meitian
authors.initials: M
authors.lastname: Wang
authors.forename: Lei
authors.initials: L
authors.lastname: Zheng
mesh_terms: D017920:Antiporters / Q000737:chemistry* / Q000378:metabolism; D002118:Calcium / Q000378:metabolism*; D027682:Cation Transport Proteins / Q000737:chemistry* / Q000378:metabolism; D018360:Crystallography, X-Ray; D006863:Hydrogen-Ion Concentration; D017136:Ion Transport; D008958:Models, Molecular
chemical_list: D017920:Antiporters; D027682:Cation Transport Proteins; C027614:calcium-hydrogen antiporters; D002118:Calcium
keywords: Ca2+ transport; H+ coupling